This application claims priority from the following Australian provisional patent applications, the full contents of which are hereby incorporated by cross-reference.
The present invention relates to a method and apparatus for forming a sheet material, and in the preferred form, a laminated sheet material.
The invention has been developed primarily for use in the formation of fibre reinforced cement (xe2x80x9cFRCxe2x80x9d) sheeting, from cementitious slurry through a modification to the xe2x80x9cHatschekxe2x80x9d process, for use in the building industry. It will therefore be described primarily with reference to this application. It should be appreciated, however, that the invention is not limited to this particular field of use, being potentially applicable to other materials, other manufacturing processes, and other industries.
The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow the significance of it to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an admission that such art is widely known or forms part of common general knowledge in the field. Sheet material, and in particular FRC sheet material, is widely used in the building and construction industries in a variety of applications including cladding, lining, framing, flooring, roofing, dooring, window framing, insulating, waterproofing, decorative trimming and the like. Depending on how the material is used in different situations, advantage is taken of its unique structural, aesthetic, acoustic, thermal, and weather resistant properties. It is typically manufactured in different sizes, shapes, thicknesses, densities and with various special purpose additives, in conjunction with other materials, so as to take optimal advantage of its functional characteristics in different applications.
FRC sheet was initially manufactured using modified paper making machinery, from cementitious slurries incorporating fibrous asbestos for reinforcement. Later, fibrillated cellulose fibre was substituted as an alternative to asbestos, and the manufacturing equipment was progressively developed more specifically to the FRC industry.
As a culmination of this development work, one of the most common manufacturing processes currently used in the industry is now known as the xe2x80x9cHatschekxe2x80x9d process. In this process, a cementitious slurry is initially formed from water, cellulose fibre, silica, cement and other additives selected to impart particular properties to the product according to its intended application. The slurry is mixed in an agitator and delivered to a feed sump from where it is pumped through a series of vats. A sieve cylinder is immersed in the slurry within each vat and these cylinders rotate as they are progressively driven by the bottom run of an overlying belt, formed from a specially formulated felt material. A typical Hatschek machine in a large scale production environment will incorporate a series of three or four vats, and a corresponding number of associated sieve cylinders. The number of vats and cylinders may vary, however, and there need not be a one to one correlation between them in the sense that several cylinders could be immersed in a single vat.
In the process, the relatively dilute slurry in the vats filters through wire mesh screens fitted to the respective sieve cylinders. As the slurry filters through this mesh, it deposits a layer of cellulose fibre on the underside surface of the wire, which acts as a filter medium to trap the other particulate materials in the feed slurry. By this mechanism, a thin film of material having a thickness of around 0.3 mm is quickly built up on the surface of the sieve. This process thickens the slurry from a concentration of around 7% solids in each vat to a concentration of around 70% solids in the film. The excess water passes through the sieve wire as filtrate and exits from the end of the sieve cylinder, so that the residual solids may be recovered and recirculated.
The film formed on the surface of each sieve cylinder is transferred upon contact to the outer surface of the overlying belt. This transfer process takes place by virtue of the fact that the felt is less porous than the sieve, as a consequence of which atmospheric pressure facilitates the transfer.
As the felt passes over each successive vat in the series it picks up a corresponding series of sequential layers of film from the associated sieves and thereafter passes over a vacuum box positioned along the top run of the belt where the accumulated layers of film on the belt have their moisture content reduced.
The layered film then passes between a tread roller, which also provides the driving force to the belt, and an adjacent accumulation or xe2x80x9csizexe2x80x9d roller in the form of a relatively large diameter drum. The tread and size rollers are positioned such that further water is pressed out of the film while it is transferred to the size roller by a mechanism similar to that by which it was previously transferred from the sieve cylinders to the belt. The size roller accumulates a number of layered films according to the number of turns allowed before the film is cut off. Thus, the formation of a thicker sheet is achieved by allowing a larger number of turns before cutting the film. In the cut off process, a wire or blade is ejected radially outwardly from the surface of the size roller to cut longitudinally the cylinder of layered film material that has cumulatively formed on the surface of the roller.
Once cut, the sheet of material peels off the size roller to be removed by a run-off conveyor. The material at this stage has the approximate consistency of wet cardboard, and therefore readily assumes a flat configuration on the run-off conveyor. To complete the process at the wet end, the felt is cleaned as it passes through an array of showers and vacuum boxes, before returning to the vats to pick up fresh layers of film. It will be appreciated that the quality and characteristics of sheet material produced from the Hatschek process are dependent upon a wide range of variables associated with the slurry formulation and the various settings at the wet end of the machine.
Further down the process line, the xe2x80x9cgreenxe2x80x9d sheet is roughly trimmed to size at a green trim station using high pressure water jet cutters, after which it proceeds as individual sheets to a stacker. At the stacker, the green sheets are picked up by vacuum pads and formed with interleaving sheets into autoclave packs.
After partial curing, and optionally a further compression process to increase density, the sheets are loaded into an autoclave unit for final curing under elevated temperature and pressure conditions. In the autoclave, a chemical reaction occurs between the raw materials to form a calcium silicate matrix which is bonded to the cellulose reinforcing fibre. This process takes around 12 hours and at its completion, the sheets emerge fully cured, ready for accurate final trimming, finishing and packing.
One of the major limitations with the Hatschek process, and other known processes for the manufacture of FRC sheet, is that because of the way in which the layers of film are progressively formed from a cementitious slurry, and because the composition of the slurry itself is critical to the formation process, it is difficult to form sheet material accurately in multiple discrete layers having substantially different material compositions. This is desirable for a number of reasons, primarily to permit a greater degree of flexibility in tailoring the structural, aesthetic and other properties of the material, so as to optimise its performance characteristics in particular applications. For example, it may be desirable to incorporate layers of fire retardant materials, textured outer layers to achieve particular aesthetic effects, softer outer layers to facilitate sanding and finishing, coloured outer layers to obviate the need for painting, or layers to modify water resistance, strength, impact resistance, thermal insulation, acoustic insulation, or other properties. In this context, various attempts have been made to introduce supplementary layers into the sheet at selected stages as it is progressively formed. To date, however, these attempts have not been successful, or at best have been only partially effective. One of the difficulties, which has not hitherto been overcome, relates to the desirability of being able to position layers of different material composition accurately at predetermined levels in the sheet. This difficulty arises partly because of the manner in which the sheet is progressively developed, partly because of the difficulty in altering the material composition or concentration within the vats during the production process, and partly because of the difficulty involved in accurately stopping and starting any sort of supplementary injection or infusion process at high speed, in synchronisation with the rest of the process.
Within the relatively rigid constraints of the existing process, it is possible uniformly to vary the overall composition of the sheet material to some extent, by using different slurry formulations in order to enhance particular selected characteristics. However, by not accommodating multiple layers of different composition, the result is often compromised in some respect either in terms of performance or cost. For example, it may be desirable to form a relatively soft outer layer on the sheet material, to facilitate sanding and finishing, whereas a sheet formed entirely from a softer formulation may be highly compromised in terms of structural integrity. Similarly, it may be that a relatively thin fire retardant layer is sufficient to substantially increase the fire rating of the sheet, whereas a sheet formed entirely from a fire retardant formulation may be prohibitively expensive.
It is possible to form a laminated product by combining one or more different layers after final curing of the FRC sheet, for example by gluing multiple sheets of different formulations together. However, this then adds significantly to the time and cost of production, and gives rise to further problems in terms of the need for development of special purpose adhesives, and the potential for warping, delamination, and the like.
In order to overcome these problems, attempts have been made to apply additional layers of slurry by incorporating supplementary application devices into the Hatschek process such that the respective layers can be integrated during the wet phase of that process. In this context, various forms of apparatus for applying liquids to substrates are known. For example, one process makes use of a spray bar whereby a liquid coating, such as a paint or primer, is squirted and atomised through spaced apart nozzles, so as to coat the substrate which typically passes progressively beneath the spray bar on a conveyor. One problem with devices of this type is that the relatively fine nozzles required to achieve the degree of atomisation necessary for uniform coating are readily clogged, particularly in the case of slurries containing a solid component in suspension. This results in inconsistent application, and requires frequent cleaning which is time consuming, costly and disruptive to the production process. Atomisation is also problematic in the case of more viscous liquids and slurries.
Another known device is a curtain coater, which makes use of a sheet or curtain of flexible fabric material which drapes over the moving substrate and applies a coating by means of a direct wiping action. However, curtain coaters are prone to inconsistent application, are not well suited for use with slurries, are limited in terms of the speed at which they can operate effectively in a production environment, and are not well adapted to applying relatively thick coatings.
Another known form of applicator is usually referred to as a flood coater, which essentially operates by forming a pool of liquid on the substrate, and spreading the pool over the surface with air jets. Again, however, there are limitations with this technique in terms of the uniformity of application, the viscosity of the liquid or slurry that can be used, and the thickness of the layer or coating that can be applied.
Generally, therefore, these known forms of apparatus are subject to a variety of limitations including susceptibility to clogging, inconsistent application, limitations in speed, limitations in the width of sheet material that the coating can be applied to, limitations in the consistency of the liquid or slurry that can be applied, or some combination of one or more of these shortcomings. They are also typically adapted to apply relatively thin outer surface coatings, as distinct from intermediate layers of substantial thickness as part of a laminated sheet. These limitations render such prior art devices generally unsuitable for use in the manufacture of sheet materials, and particularly FRC sheets, of substantial size and at relatively high speed.
Another known form of apparatus is a spatter coater, which makes use of a rotating roller incorporating a radial array of flexible filaments or bristles to spatter a coating onto a substrate. Spatter coaters are used, for example, to apply surface coatings to clay or masonry tiles, on a production line. Spatter coaters are able to some extent to overcome some of the deficiencies of the other known forms of coating apparatus, especially in terms of clogging. However, in the context of the production of sheet material, known spatter coaters are also subject to inherent limitations.
In particular, known spatter coaters are not able accurately to stop and restart the application process on an intermittent basis, in order to permit precisely controlled coating or laminating. This is especially so with the types of slurries typically used in the production of FRC sheet, because of the relatively runny consistency required to ensure xe2x80x9cself-levellingxe2x80x9d, and the consequential tendency for excess slurry to drip onto the substrate, even if the slurry supply is shut off or the spattering roller is temporarily stopped.
This precise control over intermittent stopping and starting of the application process is particularly important in a high speed production environment where different batches of sheets, having different layers, thicknesses or properties, may be required to run back to back through the coating apparatus and inaccurate transitional control can result in patchy application, or the coating for one batch running over into the following batch of product.
A further difficulty arises due to the fact that, in the manufacture of FRC sheet or other products using cementitious slurries, it is desirable not to stop the supply of slurry to the apparatus itself, as this can result in the stagnation or accumulation of slurry in the apparatus or in upstream parts of the process. This, in turn, can result in overflows, changes in slurry consistency or concentration, settling or sedimentation, or undesirable variations in other process parameters.
Known spattering apparatus do not provide for the precise interruption of the slurry application process, and also do not allow for interruption without stopping the supply of slurry to the apparatus. They are therefore not effective in enabling an accurately controlled intermittent application process, especially in a high speed production environment for sheet materials.
The foregoing description of the prior art is provided so that the present invention may be more fully understood and appreciated in its technical context and its significance more fully appreciated. Unless clearly indicated to the contrary, however, this discussion is not, and should not be interpreted as, an express or implied admission that any of the prior art referred to is widely known or forms part of common general knowledge in the field.
It is an object of the present invention to overcome or substantially ameliorate one or more of the limitations of the prior art, or at least to provide a useful alternative.
Accordingly, in a first aspect, the invention provides a method for forming a laminated sheet material from a slurry having a liquid component, said method comprising the steps of:
applying the slurry to a substrate in successive layers to form a wet sheet of predetermined thickness;
applying at least one of the layers by spattering;
removing the wet sheet from the substrate; and
drying or curing the wet sheet so as to remove at least a substantial proportion of the liquid component and thereby forming the sheet material.
The terms xe2x80x9cspatterxe2x80x9d, xe2x80x9cspatteringxe2x80x9d and the like as used herein, are intended to encompass any application technique whereby the slurry is deposited onto a surface or substrate in droplet, globule, particulate or atomised form, whether produced by brushing, flicking, rotating, spraying, agitating, atomising or other dispersion means, and whether propelled by mechanical, electrostatic, hydrostatic, hydrodynamic, gravitational or other means.
Unless the context clearly requires otherwise, throughout the description and the claims, the words xe2x80x98comprisexe2x80x99, xe2x80x98comprisingxe2x80x99, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of xe2x80x9cincluding, but not limited toxe2x80x9d.
Preferably, the slurry is a cementitious slurry, formed from a mixture of water, cellulose fibre, silica, cement and optionally other additives, in predetermined proportion according to the desired properties of the sheet material.
Preferably, the method incorporates the xe2x80x9cHatschekxe2x80x9d process, or a derivative or variation thereof, wherein the substrate takes the form of a porous belt, and the method includes the further steps of:
progressively accumulating the film on a size roller downstream of the belt until a predetermined thickness has been achieved; and
cutting and removing the accumulated material from the size roller to form the wet sheet.
Preferably, the porous belt is formed from felt, and the film is deposited at least partially on the belt using a series of sieve cylinders in rolling contact with the belt and substantially immersed in vats containing the slurry.
In the preferred embodiment, the spattered layer is formed from a material composition that is substantially different to at least one other layer in the sheet, the composition and position of the spattered layer being selected to confer or optimise predetermined physical properties or performance characteristics in the sheet. The desired properties or characteristics may include, but are not limited to enhanced water resistance, fire retardance, tensile or compressive strength, toughness, crack resistance, impact resistance, hardness, density, thickness, thermal insulation, acoustic insulation, nailability, workability, colour or surface texture.
In a preferred embodiment, the spattered slurry is a cementitious slurry and more preferably, is formed from a mixture of silica, cement, water and optionally other additives. Most preferably, the slurry is a self levelling dewaterable cementitious slurry with a solids content of between 50% and around 90%. The slurry preferably includes a dewatering aid in a sufficient quantity to permit dewatering of the slurry, preferably through the substrate with or without vacuum assistance.
Preferably, the spattered layer is applied using an apparatus for applying a slurry to a substrate, the apparatus including:
a delivery surface disposed to support a layer of slurry;
spattering means adapted to be positioned closely adjacent the delivery surface and being moveable so as to spatter the slurry from the delivery surface onto the substrate; and
regulation means for selectively varying or interrupting the flow of slurry from the delivery surface onto the substrate.
In the preferred application of the invention, the substrate will either be the porous belt, or a previously deposited film or layer of the cementitious slurry.
Preferably, the apparatus includes a reservoir to contain slurry upstream of the regulation means. Preferably, the reservoir includes an inlet to direct slurry from a supply source, and an outlet associated with the regulation means.
Preferably, the reservoir includes an inlet to direct the slurry from a supply source and an outlet associated with the regulation means. The regulation means preferably include a pair of barrier elements selectively moveable to define an intermediate clearance space of variable effective cross sectional flow area, thereby to permit selective regulation of the flow rate of the slurry between the barrier elements. Preferably, the barrier elements are adapted in a closed configuration to selectively shut off the flow of slurry between the reservoir and the delivery surface.
Preferably, one of the barrier elements is a first cylindrical roller rotatable about a first axis. The other of the barrier elements is preferably a second cylindrical roller rotatable about a second axis, substantially parallel to the first. The rollers are preferably configured to rotate in opposite directions.
The first roller preferably takes the form of a delivery drum, the outer surface of which constitutes the delivery surface. The second roller preferably takes the form of a metering roller, selectively moveable toward and away from the delivery drum, so as to permit selective variation or interruption of the spattering process, as part of the regulation means.
In one embodiment, the reservoir is at least partially defined by a tank positioned immediately above the delivery and metering rollers. In an alternative embodiment, the reservoir is simply a containment region defined between adjacent rollers, preferably the delivery roller and an abutting idler roller, with the metering roller being positioned above the delivery roller.
The apparatus preferably further includes a main frame supporting the delivery drum, and a first sub-frame on which the metering roller is mounted. The first sub-frame is preferably rotatable about a third axis, substantially parallel to and spaced apart from the second axis, whereby rotation of the first sub-frame about the third axis adjustably displaces the metering roller towards and away from the delivery roller.
In one preferred embodiment, the apparatus includes hydraulic or pneumatic actuation means for adjustably moving the metering roller and the delivery drum toward and away from each other.
Preferably, the spattering means include a generally cylindrical body rotatable about a fourth axis, generally parallel to the other axes, and a plurality of resiliently flexible elongate spattering elements extending radially outwardly from the body. The body and spattering elements together preferably form a spattering roller.
The apparatus preferably includes a second sub-frame on which the spattering roller is mounted, the second sub-frame being rotatable about a fifth axis, generally parallel to and spaced apart from the fourth axis, to permit adjustable displacement of the spattering roller toward and away from the delivery drum.
Preferably, the spacing between the various rollers, as well as the speed of the rollers, are selectively adjustable to permit regulation of the spattering rate according to the speed of the production process, the desired thickness of the spattered layer, the consistency of the spattered slurry, and other relevant parameters.
In one particularly preferred embodiment of the invention, the method incorporates a series of spattering apparatus disposed to operate on the same Hatschek machine, with vacuum boxes optionally being positioned after each spattering apparatus to facilitate dewatering through the sheet. In this arrangement, each spattering apparatus may be configured to deliver slurry formulations having either identical or different compositions, corresponding to desired aesthetic, functional or performance characteristics in the finished sheet. It will be appreciated that the array of spattering apparatus may be controlled to deliver single or multiple layers between successive fibre cement laminates.
Although the invention has described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.